EncodeString: Restore dependency on vtksys
The dependency was dropped by commit 708c3a5c (EncodeString: Compile independently from other VTK modules, 2015-12-04), but it is still needed to support VTK_BUILD_FORWARDING_EXECUTABLES.
The dependency was dropped by commit 708c3a5c (EncodeString: Compile independently from other VTK modules, 2015-12-04), but it is still needed to support VTK_BUILD_FORWARDING_EXECUTABLES.